envelopes
nounUK /ˈɛnvələʊps/
US /ˈɛnvəloʊps/
Flat paper covers that you put letters or cards inside before you mail them.
She sealed the letters in envelopes and put them in the mailbox.
RELATED WORDS
OTHER MEANINGS
2. The outer coverings or layers that surround something.
The building's glass envelopes keep the heat inside.
3. The limits or boundaries within which something operates.
The team pushed the envelopes of what was possible.
4. In math, flat curves that wrap around and touch a family of lines or curves.
The engineer studied the envelope of the moving curve.
5. The outer protective covering of a balloon or airship.
The balloon's fabric envelope slowly filled with gas.